Conventional variable-resistance conductive elastomers contain a conductivity additive, such as metal particles, carbon fibers, graphite particles or semiconductor particles, mixed with and dispersed into a non-conductive elastomer. The non-conductive elastomers include natural rubber, synthetic rubbers such as chloroprene rubber, ethylene propylene diene rubber (EPDM), acrylonitrile rubber, styrene butadiene rubber (SBR) and silicone rubber, and thermoplastic elastomers having rubber-like elasticity.
Examined patent applications Nos. SHO 56-9187 and SHO 56-54019 disclose a composite produced by blending a base of a rubber or a flexible organic material with conductive particles of pebbly graphite having their edges rounded off. Unexamined patent application No. SHO 62-11264 discloses a composite produced by blending a base having the elasticity of an insulating rubber with conductive particles which are produced by burning and carbonizing spherical and polymeric materials. These composites, which are simply conductive particles mixed into an elastic insulating base, have the problem that they are hard, fragile and weak in impact resistance. In addition, since they are designed to respond only to compression deformation, they are inferior in mechanical strength and their resistance values may not change adequately when deformed by extension.
Examined patent applications Nos. SHO 60-722 and SHO 60-723 disclose a composite having a foamed structure produced by dispersing conductive metal particles into a liquid silicone rubber base. The foaming agent is an organic compound, such as n-propyl alcohol or n-butyl alcohol, whose parameter of solubility is more than 9.8, or an organic compound having an N-nitroso group. While an improvement in impact resistance results when the composite is foamed, the composite remains inferior in mechanical strength and durability against repeated impact. Since foamed composites are designed to respond to compression deformation, they have the disadvantage of being poor in mechanical strength and show little change in resistance when deformed by extension. Also, when a volatile alcohol is used as the foaming agent, mixing and blending of the conductive particles into the silicone rubber is very difficult. And when an organic compound having a N-nitroso group is used as the foaming agent, the cells formed on foaming are not uniform, and the composite is not uniform in its properties.
Unexamined patent application No. SHO 61-80708 discloses a composite produced by blending a base comprising a polymeric elastomer with a thin-piece conductive filler produced by plating a thin piece of mica or glass with nickel or copper. The composite is capable of making an adequate response on extension deformation. However, the thin-piece conductive filler, which is arranged in parallel to the direction of extension, causes the composite to be hard and fragile, and to have inferior mechanical strength and low resistance to impact. In addition, because it is designed only to respond to extension deformation, it does not respond adequately on compression deformation. And since the conductive filler is thin, it does not respond well to deformation by bending or twisting.